What are "Leaf" bones? (Collada import question)
I'm not versed in armatures or rigging, nor an expert in that matter, but a little Google research yielded some results about it.

From the Makehuman bug tracker we can learn that apparently most applications and file formats don't store bone length information. This is normally not an issue, since bone length is rarely a needed information. In most cases it can be extrapolated from child bones.

A bone normally extends from its head until it's child's head, length is implicit and can be calculated from the distance between these two points.

It can however represent an issue for end bones, on the extremities of the armature, where no child bones are present.

On such cases an extra fake dummy bone can be created exclusively to represent the end of its parent, and no other function of its own.

These loose bones at the end of an armature are then called "Leaf Bones" by analogy of a tree leaf, I assume.